Thanks TCS, but the drives had worked perfectly for at least a year and then developed the problem with no changes made to the system hardware. There is a CD-RW as the slave drive on the same port in one of the systems. I'm 99.999% sure the jumper is right - why else would it have worked OK for so long?
In one case I think the problem appeared immediately after I pressed the eject button on the drive during a system boot-up. I don't recall what had preceeded the problem with the other CD-ROM drive. It was removed and has been sitting on the shelf for a long time.
Tnx anyway,
sc